This invention relates to electrostatic precipitators and particularly to a method of protecting the wire electrodes from erosion or "burn-off" near their lower ends where electrical sparking tends to occur.
In electrostatic precipitators of the type used to remove fly ash from the flue gas of a coal fired boiler, the discharge electrodes are, typically, 0.093-0.109 in. stainless steel wires which are vertically suspended from frame members. The wires are generally 25-40 feet long and hang freely between the grounded plate electrodes. The wires have weights attached to hook portions formed at their lower ends to keep them taut and prevent them from being blown closer to one electrode plate than the other by the force of the flue gas stream which moves horizontally between the plates.
Wire failures of the discharge electrodes can result from a number of different reasons, all of which are not operational. For instance, if a wire is installed kinked, or the alignment of the wires between the collecting plate is poor, the wires will erode as a result of successive power arcs to the same point on the wire. This point is usually that sector of the wire that passes nearest the grounded plate. Proper maintenance can usually revolve this situation. On the other hand, there are precipitator applications where the process itself is conducive to precipitator wire failure. These applications normally consist of low sulfur coal fired boilers with precipitators that operate at predominantly higher spark rates. On units of this sort, the higher resistive load occurs at the bottom of the unit where heavier dust concentrations exist. As a result of this and also the location of lower plate stiffeners, the electrical sparking that does occur will occur at this point. The continued succession of power arcs to the same point on the wire results in erosion or "burning off" of the wire at that point. A typical wire that has burned off as a result of this effect will have an end that comes to a needle point at a position on the wire that is opposite the lower plate stiffener.
On newer low sulfur applications, it is common practice to provide discharge electrodes with heavier metal cross sections at the top and bottom or just bottom, as the case may be, to accommodate the heavier sparking. The affect of the "shroud" as it is called, is to (1) provide a heavier wire construction at the more vulnerable locations in order to accommodate any electrical erosion that may occur and (2) the increased diameter of the wire as a result of the shroud will cause the corona concentrations to be much less than the remainder of the wire and thus discourage any electrical arcs from taking place.
Although precipitator vendors recognize the need for the shrouded wires on these newer applications and provide them on the original installation, there are many installations that were originally designed and operated on high sulfur fuel that were not provided with the shrouded wires. These installations have since converted to low sulfur fuel and are now experiencing wire failures. As a result of going to low sulfur firing, the aforementioned higher spark rates are resulting in excessive wire failures and inconveniences to the operators. In order to eliminate the nuisance and downtime resulting from wire failures, operators are forced into completely replacing all existing wires with the shrouded wires at a considerable expense to themselves. One alternative that has been tried in the past is installing metal clip-on shrouds to the existing wires. Vendors have had little success with this method in that if the metal shroud is not installed correctly, fly ash will gather between the shroud and the wire, isolating them electrically. When this occurs, the shroud takes on a different potential than the wire and current flow or arcing takes place. In this case, the shroud is the direct cause of the wire failure. For this reason, it is usually best that the existing wires be replaced with new shrouded wires that have been prepared at the factory. These have a very good success rate but pose considerable expense to the operator. Obviously, it would be desirable to be able to reduce the very considerable time and expense of replacing the discharge wires since a single precipitator can contain 10,000 or more wires.